/Engine/Source/Runtime/Engine/Classes/Engine/World.h
This function allows pausing the seamless travel in the middle, right before it starts loading the destination (i.e. while in the transition level) this gives the opportunity to perform any other loading tasks before the final transition this function has no effect if we have already started loading the destination (you will get a log warning if this is the case)
void SetSeamlessTravelMidpointPause ( bool bNowPaused )